As is known, turbine engines in aeronautical applications have an air/oil separator comprising a toroidal-shaped filter, which is mounted on a rotor and is defined by a series of ring-shaped filter elements, made from a metal foam. The filter receives an incoming mixture of air and oil on one of its faces and allows the air to pass through, trapping the oil particles in the pores in the metal foam. Due to the effect of the rotation of the rotor the oil is made to pass radially through the pores, before being discharged and flowing out towards a tank.
The production of filter elements made from nickel-chrome or nickel alloys is known, using a matrix that consists of spheres of polymeric material, the positions and dimensions of which correspond to those that the pores in the metal foam will have. The porous structure is obtained by laying the metal alloy (for instance using electro-chemical means) in the spaces between the spheres and subsequently eliminating the polymeric material.
According to a different process, the production of filter elements made from an aluminium alloy is known. In this case, the pores in the metal foam are obtained by blowing gas into the alloy while the latter is still in the liquid state.
The filter elements obtained using the known methods described above are not entirely satisfactory, in that the centrifugal inertia forces exerted on the filter due to the rotation of the rotor are not balanced. With the known production methods the metal material is not distributed in a perfectly uniform manner in the volume of the filter elements.
In particular for processes using a polymeric matrix, this problem is due to the fact that the position of the pores is determined by the position of the spheres in the polymeric material, which is in turn not entirely controllable and thus has a certain degree of randomness.
Even slight non-uniformities in the distribution of the material and, thus, slight deviations of the centre of gravity of the filter with respect to the axis of the rotor cause problems in terms of inertia forces, as the rotor turns at high speeds (up to approx. 18,000 rpm).
Moreover, also due to the randomness of the position of the pores, the various filter elements are not perfectly identical to one another, which means that different air/oil separators might behave slightly differently in turbine engines, at the same speed of rotation.